The Oak tree is a symbol of strength and longevity and considered the king of trees. It's wood is hard and can be used in many ways.
It was particularly respected in ancient times. In Celtic mythology it is a gateway between worlds. In classical mythology it was sacred and in popular culture it serves as a symbol. The Royal Oak of King Charles II and the Major Oak of Robin Hood and the Hearts of Oak tradition. Romans used oak leaf crowns for victory and oak leaves are still used to symbolise special mention in military campaigns.
Pretty much regarded as a sacred tree all over Europe it holds a special place in folk memory. Druids practised in Oak groves which gave them the Mistletoe - a mysterious plant that grew on the Oak in Winter. It is associated with Thor, Zeus, Jupiter, Thoth, Dagda and Rhianon in cultures spanning Egypt, Greece, Celts, Romans and Scandinavia.